The Player's Handbook on Sustained Durations, in part, says, "Starting on the turn after you create an effect, you sustain the effect by taking the indicated action: a standard action, a move action, or a minor action. (You can sustain an effect once per turn.)" (278). Likewise, the Rules Compendium on Sustain, in part, says more succinctly, "The creature cannot take this sustaining action until its turn after it uses the power and can take the action no more than once per round" (97).
So, yeah, unless a game element makes an exception (and I welcome comments offering examples), you just can't sustain on the turn you activate.
…But that's okay here
The Player's Handbook on Saving Throws, in part, says, "End of Turn: At the end of your turn, you make a saving throw against each effect on you that a save can end" (279). The Rules Compendium on When to Make Saving Throws rephrases but doesn't change this: "If a creature is subject to an effect that a save can end, the creature makes a saving throw against that effect at the end of each of its turns" (228).
The Sustain Minor entry of the power crown of madness (PH 134) says, "The target makes a melee basic attack against one of its adjacent allies of your choice (save ends)." However, until the PC ("crowner") sustains the power, the foe ("crownee") makes no saving throws against it. To summarize:
- Round 1: On the crowner's turn, the crowner uses the crown of madness power. On the crownee's turn, the crownee takes its turn normally.
- Round 2+: The crowner sustains the crown of madness power and—still on the crowner's turn—the crownee makes an attack against its ally. The crownee takes its turn normally, but, at the end of the crownee's own turn, the crownee makes a saving throw against the crown of madness power's Sustain Minor effect.
Success on the saving throw means that the Sustain Minor effect of the crown of madness power ends, and failure means that the crowner on the crowner's next turn can Sustain Minor again, causing the crownee again to attack its ally. This continues until the crowner opts not to sustain or until the crownee succeeds on the saving throw.
In short, the crown of madness power always deals damage (even if it's half on a miss), and, if it's sustained, always makes the foe attack a friend (within the effect's guidelines, of course). That said, while still not as good as it could be, I think it's at least as good as you thought it should be.